Bond wire vibration fatigue failure is a problem for certain automotive electronic devices, including, but not limited to, various types of sensors, such as pressure sensors.
For example, FIG. 1 depicts a transmission-fluid pressure sensor 100 in accordance with the prior art. The transmission-fluid pressure sensor includes a housing 102. The bond wires for electrical connections are located in a pocket 104 with a high density protection gel encapsulating them. During operation of an automobile, bond wires of this type may encounter vibration fatigue failure.
FIG. 1 also depicts a cover 106 for the pocket 104. The cover includes a rim 108.
FIG. 2 is a top view of the pocket 104. Eighteen bond wires 202 are shown in FIG. 2. Also in the pocket are printed circuit board (PCB) 204, application-specific integrated circuits (ASICs) 208 and 210, chip capacitors 206, and pressure sensing elements 212.
FIG. 3 is a cross-sectional view, along sectional view line I-I of FIG. 2, of the housing pocket 104 and the cover 106. The pressure sensing element 212 is electrically coupled to the ASIC 210 by bond wires 202-1, which are attached to bond pads on the pressure sensing element 212 and on the ASIC 210. Similarly, the ASIC 210 is electrically coupled to the PCB 204 by bond wires 202-2, which are attached to bond pads on the ASIC 210 and the PCB 204. And the PCB 204 is electrically coupled to a lead frame 318 by bond wires 202-3, which are attached to bond pads on the PCB 204 and to the lead frame 318. Chip capacitors 206 are used for electromagnetic compatibility (EMC) by coupling electrical signals between the wires 202-2 and 202-3 through metal traces (not shown) in the PCB 204 and electrically conductive adhesive (ECA) or solder 316 on the PCB 204.
Other components shown in FIG. 3 include: a glass cap 302, a vacuum cavity 304, a glass pedestal 308, gel 306, transmission-fluid pressure 310, ambient pressure 311, epoxy 312, and adhesive 314.
FIG. 4 is a log-log-scaled graphical depiction of the power spectral density impressed on a bond wire of a transmission-fluid pressure sensor by random vibration as a function of the frequency of wave fronts. FIG. 4 shows a random vibration example in a log-log scale. As the frequency increases above a critical value F1, the power spectral density on a bond wire increases linearly to a second frequency F2. At frequencies above F2, random-vibration-induced power spectral density gradually decreases linearly.
FIGS. 5A and 5B depict pressure or forces on bond wires from vibration-induced wave fronts. FIGS. 5A and 5B are graphical depictions of the forces exerted on a “long” bond wire and a “short” bond wire. Lateral forces from the wave fronts are distributed over the length of the wire and represented in the figures by the arrows identified by “F.” While the wave fronts can strike the wires at any angle, the force that is orthogonal to the wire's axial length is the force that tends to break the wire and/or its bond due to the lateral displacements D1 and D2 that a force normal to the wire's axis tends to cause.
As is well known, the total force F exerted on a surface of area A, by a pressure of magnitude P acting uniformly over the entire area, is the product of P and A. In other words,F=P*A 
where F is the force on an area A under a uniform pressure P.
In the pocket 104, since the gel edge is considered herein to be essentially “anchored” to the sidewall, the wave front pressure P in the horizontal direction is proportional to the gel acceleration a multiplied by the gel density ρ times the “width” of the gel W, which is the width of the pocket 104. Stated another way,P∝ρ×a×W 
where ρ is the density of the gel 306, a is the acceleration of the gel and W is the “width” of the gel, perpendicular to the wire inside the pocket 104.
FIGS. 5A and 5B show that for a given wave-front pressure, the total force exerted on a “long” bond wire will be greater than the total force exerted on a “short” bond wire. The wave fronts that strike long bond wires thus tend to cause such wires and/or their connections to fail.
A method and/or apparatus to reduce vibration-induced wave fronts in a pocket 104 containing the gel 306 would be an improvement over the prior art in that reducing wave fronts would tend to reduce bond wire failure as well as reduce bond wire connection failure.